NASA's Hubble Space Telescope has recently
obtained images of the planetary nebula NGC
6369. This object is known to amateur
astronomers as the "Little Ghost Nebula,"
because it appears as a small, ghostly cloud
surrounding the faint, dying central star. NGC
6369 lies in the direction of the constellation
Ophiuchus, at a distance estimated to be
between about 2,000 and 5,000 light-years from
Earth.

When a star with a mass similar to that of our
own Sun nears the end of its lifetime, it expands
in size to become a red giant. The red-giant stage
ends when the star expels its outer layers into
space, producing a faintly glowing nebula.
Astronomers call such an object a planetary
nebula, because its round shape resembles that
of a planet when viewed with a small telescope.

The Hubble photograph of NGC 6369, captured
with the Wide Field Planetary Camera 2 (WFPC2)
in February 2002, reveals remarkable details of
the ejection process that are not visible from
ground-based telescopes because of the blurring
produced by the Earth's atmosphere.

The remnant stellar core in the center is now
sending out a flood of ultraviolet (UV) light into
the surrounding gas. The prominent blue-green
ring, nearly a light-year in diameter, marks the
location where the energetic UV light has stripped
electrons off of atoms in the gas. This process is
called ionization. In the redder gas at larger distances from the star,
where the UV light is less intense, the ionization process is less
advanced. Even farther outside the main body of the nebula, one can
see fainter wisps of gas that were lost from the star at the beginning
of the ejection process.

The color image has been produced by combining WFPC2 pictures
taken through filters that isolate light emitted by three different
chemical elements with different degrees of ionization. The
doughnut-shaped blue-green ring represents light from ionized
oxygen atoms that have lost two electrons (blue) and from hydrogen
atoms that have lost their single electrons (green). Red marks
emission from nitrogen atoms that have lost only one electron.

Our own Sun may eject a similar nebula, but not for another 5 billion
years. The gas will expand away from the star at about 15 miles per
second, dissipating into interstellar space after some 10,000 years.
After that, the remnant stellar ember in the center will gradually cool
off for billions of years as a tiny white dwarf star, and eventually wink
out.

November 7, 2002
Credit: NASA and The Hubble Heritage Team (STScI/AURA)